Locking wrenches or pliers have been commercially available, for example, under the name of "hand vice" and are generally of the structure including a tool body formed rigid with a stationary jaw, a movable jaw pivotally connected with the tool body for movement toward and away from the stationary jaw, an operating handle pivotally secured at its basal end to the movable jaw, an adjusting bolt threadably fitted to the tool body, and a toggle link pivoted at its inner, operating end to the operating handle and bearing at the other end against the inner end of the adjusting bolt. In use, when the operating handle is gripped by hand together with the tool body after the movable jaw has been adjusted by operation of the adjusting bolt into an appropriate position relative to the stationary jaw for thickness of the object to be gripped therebetween, the toggle link is laterally forced so as to be compressed between the adjusting bolt and the operating lever, to which the toggle link is pivoted, as will be described later in more detail, so that the stationary and movable jaws are urged toward each other to firmly grip the object.
This type of wrench is highly valued for its extraordinarily large force of grip but involves a disadvantage that a troublesome bolt-adjusting operation is required each time the thickness of the object to be gripped changes. In addition, if mishandled, its force of grip might be excessively increased to cause jaw breakage. This incurs a serious danger particularly in cases where the tool is used at levels high above the ground.